Packers are used downhole to isolate zones in a wellbore. Many styles of packers are in use depending on the application and well conditions. A common design uses an annularly shaped sealing element that is axially compressed by setting down weight, or a setting tool that holds a mandrel and pushes down on a setting sleeve or a hydraulic mechanism that involves blocking a path through the packer and building pressure on a piston assembly to compress the sealing element. When the sealing element is compressed axially it extends radially into a sealing relationship with the surrounding tubular. To enhance the grip of the extended element there is also an upper and a lower set of slips disposed on opposed sides of the sealing element. The slips generally comprise tapered segments with exterior wickers that bite into the surrounding tubular when ramped out on tapered surfaces during the process of axially compressing the sealing element.
One issue with the compression set sealing elements is extrusion in the uphole or the downhole directions. Frequently, anti-extrusion rings are placed at the opposed ends of the sealing element. They plastically deform when the sealing element is axially compressed and engage the surrounding tubular to create a barrier at opposed ends. The problem with anti-extrusion rings is when the packer is retrieved. The plastically deformed rings retain their deformed shape despite extension of the packer mandrel assembly that allows the sealing element to extend axially and radially retract. In essence, the backup rings can still be in contact with the surrounding tubular after the sealing element has retracted away from the backup rings in a radial and an axial direction. When the packer is pulled out in this condition, the backup rings can swab the well as the packer is removed. Swabbing is the act of reducing pressure by removal of a tool that seals as it is being retrieved. This swabbing can cause formation damage or lead to the well coming in and a potential loss of well control. Also, well fluid above the packer is displaced upward or through a small bypass in the tool. This condition severely limits retrieval speed. Another problem is that the backup rings can get mangled on the trip out of the hole and cause the packer to hang up and in severe cases the packer may have to be milled to remove it.
Traditional designs have slips above and below the sealing element. A problem with this design is that when in service, and exposed to pressure differentials acting on the mandrel with the packer set there is a transfer of the applied pressure differential to the wickers of the uphole slips if the differential pressure is in the uphole direction and on the downhole slips if the pressure differential is in the downhole direction. This arrangement creates added stress on the surrounding tubular from the force increment on the slips created by the applied pressure differential.
There is yet another issue with debris in the well such as sand or gravel settling on top of the anti-extrusion rings, thus making it difficult to extract the packer after release.
Extrusion barriers different from continuous pliable rings that plastically deform have been tried. The idea behind a segmented ring design is the ability to maintain an overlapping relationship of the segments as they are ramped out on a tapered surface. This design is illustrated in U.S. Pat. No. 7,290,603. The problem with this design that used long return springs in the hope of biasing the segments to retract is twofold. The long spring members are exposed and can get damaged during run in. The debris in the well can get on the ramp surface or under the long spring elements and prevent the segments from retracting. This design also transfers load from differential pressure into the slips to increase stress in the surrounding tubing wall.
What is needed is an anti-extrusion system that is protected from well fluid debris after it is set while also minimizing the forces created from pressure differentials while in service from further stressing the surrounding tubular. An improved retraction system for a fully circumferential extrusion barrier is also provided to a barrier shielded from well fluids between seals. The barrier elements can have external wickers and function as slips as well as a barrier. The elements can also have a ring segment mounted to their wide dimension where the ring segments span over the region where the elements move relatively in the axial direction to change diameter. In the gripping position the seal is further isolated from exposure to relatively moving segments that can damage the seal. These and other features of the present invention will become more readily apparent to those skilled in the art from a review of the description of the preferred embodiment below along with the associated drawings, while recognizing that the full scope of the invention is to be found in the literal and equivalent scope of the appended claims.